第二人生的源码分析(二十六)底层网络协议

为了理解第二人生的客户端与服务器的沟通，那么下面就来分析一下第二人生采用的网络协议。在目前的网络里，主要有两个协议：TCP和UDP，而第二人生里是采用UDP协议。TCP协议与UDP协议的主要区别，就是TCP有流量控制，可靠性控制，IP层组包功能，连接需要三次握手，而UDP没有这些保证，因此UDP发送的数据包需要自己来管理数据的有序性和可靠性。先来分析最底层的协议，它调用UDP发送数据的源码如下：

#001 // Returns TRUE on success.

#002 BOOL send_packet(int hSocket, const char *sendBuffer, int size, U32 recipient, int nPort)

#003 {

#004      // Sends a packet to the address set in initNet

#005      // 

#006      int nRet = 0;

#007      U32 last_error = 0;

#008 

#009      stDstAddr.sin_addr.s_addr = recipient;

#010      stDstAddr.sin_port = htons(nPort);

#011      do

#012      {

#013             nRet = sendto(hSocket, sendBuffer, size, 0, (struct sockaddr*)&stDstAddr, sizeof(stDstAddr));                

#014     

#015 

#016             if (nRet == SOCKET_ERROR )

#017             {

#018                    last_error = WSAGetLastError();

#019                    if (last_error != WSAEWOULDBLOCK)

#020                    {

#021                           // WSAECONNRESET - I think this is caused by an ICMP "connection refused"

#022                           // message being sent back from a Linux box... I'm not finding helpful

#023                           // documentation or web pages on this. The question is whether the packet

#024                           // actually got sent or not. Based on the structure of this code, I would

#025                           // assume it is. JNC 2002.01.18

#026                           if (WSAECONNRESET == WSAGetLastError())

#027                           {

#028                                  return TRUE;

#029                           }

#030                           llinfos << "sendto() failed to " << u32_to_ip_string(recipient) << ":" << nPort

#031                                  << ", Error " << last_error << llendl;

#032                    }

#033             }

#034      } while ( (nRet == SOCKET_ERROR)

#035                     &&(last_error == WSAEWOULDBLOCK));

#036 

#037      return (nRet != SOCKET_ERROR);

#038 }

 

在13行里调用WINDOWS API函数sendto来发送数据sendBuffer到目标地址stDstAddr里，这样就可以把数据交给WINDOWS的网络去发送数据了。由于这里采用异步的UDP来发送数据，因此采用了一个do while来处理正在阻塞的情况。当发送不成功，并且错误码是WSAEWOULDBLOCK，说明WINDOWS API正在发送过程中，不能接收数据，这样就循环地检查，直到发送成功，或者出错为止。第二人生里跟服务器发送的消息，就是这样发送出去的，看起来比较简单吧。

 蔡军生 2008/3/20 QQ:9073204 深圳